1. Field of the Invention
The present invention relates to a brake pressure control apparatus for automobiles with an anti-lock braking system, and more particularly to a brake pressure control apparatus for automobiles with an anti-lock braking system which is easily operated and has a simple construction.
2. Prior Arts
Generally, an anti-lock braking system (ABS) is used for preventing the wheels of the automobile from "locking" in the event of a sudden stop of the automobile. The term "locking of the wheels" means that the rolling wheels of the automobile are stopped by the brake pressure applied to the wheels when a driver puts on the brake suddenly during driving the automobile. When the wheels are subjected to this type of locking condition, the wheels slip toward the running direction due to the inertia force of the vehicle so that the frictional force between the wheels and the road surface may be reduced. For this reason, the braking distance may become longer and steering the vehicle may be impossible, thereby causing fatal accidents.
In order to prevent these types of accidents, an anti-lock braking system is provided to the vehicle. The anti-lock braking system increases, maintains, and reduces the brake pressure applied to the wheels rapidly and repeatedly so that the locking of the wheels may be prevented, thereby preventing the fatal accidents.
Generally, the ABS comprises a valve system operated by a pressure source, such as a hydraulic pump according to an electrical signal so as to increase, maintain and reduce the brake pressure to the wheels, a sensor for sensing the RPM of the wheels and a control unit for operating the ABS according to a predetermined algorithm.
In the ABS mentioned above, the first step wherein the brake pressure to the wheels increases is called as a pressure increasing mode, the second step wherein the brake pressure to the wheels maintains constant is called as a pressure maintaining mode and the third step wherein the brake pressure to the wheels reduces is called as a pressure reducing mode.
In the conventional ABS, the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode are performed by operating a solenoid valve.
One of conventional solenoid valves used in the ABS is shown in FIGS. 4A-4C. FIGS. 4A-4C show the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode of the solenoid valve, respectively.
As shown in FIGS. 4A-4C, a solenoid valve 500 includes a cylindrical valve body 510 and a cover 520 attached to a lower end of cylindrical valve body 510 for preventing the leakage of pressure therefrom. Cylindrical valve body 510 has a first port 512 connected to a wheel cylinder (not shown), a second port 560 through which the pressure fluid circulates to a pump (not shown), a first chamber 516 formed above second port 560 and a second chamber 518 formed below second port 560.
An upper valve seat 550 having a pressure fluid inlet 552 is provided at the upper end of first chamber 516 and a lower valve seat 560 having a pressure fluid outlet 564 is provided at the lower end of first chamber 516. A cylinder 558 is disposed between upper valve seat 550 and lower valve seat 560. First and second balls 554 and 562 which are respectively in contact with upper and lower valve seats 550 and 560, are provided at the upper and lower ends of cylinder 558, respectively. First and second springs 556 and 566 are accommodated in cylinder 558 in such a manner that first and second springs 556 and 566 can elastically support first and second balls 554 and 562, respectively. Between first and second springs 556 and 566, a head portion 534 of a movable plunger 532 is disposed so that head portion 534 of plunger 532 pushes first spring 556 when plunger 532 moves upwards. Thus, pressure fluid inlet 552 of upper valve seat 550 is closed by first ball 554.
Second chamber 518 of valve body 510 includes an armature 530 securely coupled to one end of plunger 532, a bush 542 inserted in the upper end portion of second chamber 518 so as to guide the movement of plunger 532 and a solenoid 540 which applies the magnetic force to armature 530 so as to move armature 530 upwards. Between armature 530 and the underside of lower valve seat 560, a third spring 576 for elastically supporting armature 530 is disposed.
The conventional solenoid valve having the above structure is operated as follows.
When a driver puts on the brake suddenly, the speed of the vehicle suddenly decreases. At this time, a speed sensor (not shown) attached to the wheels of the vehicle senses the decreased speed and then sends an operating signal to an electrical control unit (ECU, not shown). Then, the ECU operates the ABS according to a predetermined algorithm. That is, the brake pressure applied to the wheels of the vehicle continuously increases (the pressure increasing mode), maintains constant (the pressure maintaining mode) and reduces (the pressure reducing mode) in accordance with the predetermined algorithm of the ECU.
In the pressure increasing mode, as shown in FIG. 4A, the ECU operates a hydraulic pump (not shown) so that the pressure fluid flows into pressure fluid inlet 552 from the hydraulic pump while pushing first ball 554 downwards. Then, the pressure fluid that has flowed into pressure fluid inlet 552 is applied to the wheel cylinder connected to the wheels through first port 512 so that the brake pressure to the wheels increases.
Then, an electric power is applied to solenoid 540 to generate magnetic power. At the same time, armature 530 moves upwards by the magnetic power of solenoid 540 while overcoming the bias force of third spring 576.
When armature 530 moves upwards, plunger 532 securely inserted in armature 530 also moves upwards while pushing first spring 556 upwards. Therefore, as shown in FIG. 4B, pressure fluid inlet 552 is closed by first ball 554.
From this state, the pressure fluid does not flow into solenoid valve 500 so that solenoid valve 500 maintains the pressure maintaining mode, wherein the constant brake pressure is applied to the wheels.
On the other hand, when the electric power is continuously applied to solenoid 540, as shown in FIG. 4C, armature 530 moves upwards to the underside of bush 542 inserted in the upper end portion of second chamber 518 of valve body 510. Second ball 562 rested in lower valve seat 560 is simultaneously pushed by a neck portion 536 of plunger 532 so that second ball 563 moves upwards. At this time, the pressure fluid leaks through a fine gap formed between pressure fluid outlet 564 of lower valve seat 560 and plunger 532 and then, the leaked pressure fluid is exhausted to a fluid tank (not shown) through second port 514. From this state solenoid valve 500 maintains the pressure reducing mode wherein the brake pressure applied to the wheels of the vehicle reduces.
The conventional solenoid valve rapidly repeats the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode in a short time period so that the locking of wheels can be prevented when the driver puts on the brake suddenly.
However, since the conventional solenoid valve requires various elements, the construction of the conventional solenoid valve may be complicated. Therefore, controlling the conventional solenoid valve is difficult. Moreover, the cost for manufacturing the conventional solenoid valve increases.
Further, the first, second and third springs must be manufactured accurately for performing the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode of the solenoid valve precisely. Thus, manufacturing the solenoid valve is difficult.